Is the recipient’s server at fault?

No. Their server just lacks BDAT support, which is allowed. The bad line endings come from the sending app.

Why do only some recipients bounce?

Recipients whose servers support CHUNKING get the message via BDAT, which can carry it. Only DATA based routes fail.

5.6.11: Microsoft rejects bare linefeeds in message

5.6.11Bare linefeeds are illegalPermanent: retrying will not helpFrom Microsoft 365 and Outlook.com

Example

554 5.6.11 SMTPSEND.BareLinefeedsAreIllegal; message contains bare linefeeds, which cannot be sent via DATA

What it means

SMTP requires every line to end with CRLF (RFC 5321). A bare linefeed is a lone LF character with no CR in front. Exchange can carry such content internally, but when it relays the message to a server that does not advertise BINARYMIME and CHUNKING it has to use the DATA command, and DATA cannot carry bare linefeeds. Exchange stops and returns 5.6.11.

So the message is broken on the sending side, but the problem only shows up for certain recipients: those whose servers do not support the BDAT extension. That is why mail from the same app reaches Microsoft 365 users and bounces for others.

RFC 3463 uses X.6.x for media errors. Microsoft’s 5.6.11 detail is its own, and the SMTPSEND prefix tells you the failure happened in Exchange’s sending component.

Likely causes

A script or app generates the message body with Unix line endings (LF) and submits it to Exchange.
A scanner, printer, or line of business app builds MIME by hand without CRLF.
An attachment is sent as raw text instead of being base64 encoded.
A third party relay passes the content through unchanged into Exchange Online.

How to fix it

Fix the sending app so every line in headers and body ends with CRLF; most mail libraries do this when you use their MIME builder.
Encode binary or text attachments as base64 rather than inserting them raw.
Test by sending from the app to an external address on a server that does not offer CHUNKING.
As a stopgap, send through a relay that normalises line endings before Exchange.

Checks that apply

Your domain or sending IP (fills in the checks below)

SMTP Diagnostics Shows the EHLO extensions the recipient’s server offers; no CHUNKING or BINARYMIME means Exchange must use DATA.
Email Header Analyzer Shows which client or mailer header generated the message so you know which app to fix.

FAQ

Is the recipient’s server at fault?: No. Their server just lacks BDAT support, which is allowed. The bad line endings come from the sending app.
Why do only some recipients bounce?: Recipients whose servers support CHUNKING get the message via BDAT, which can carry it. Only DATA based routes fail.

Microsoft 365 delist portal Outlook.com SNDS

All 191 bounce codes